1. Field of the Invention
The present invention relates to a circuit and method for improving EMI noise performance in a bridgeless PFC boost converter, and more particularly to an improvement including a bypass filter capacitor connected to an AC source terminal.
2. Related Art
A conventional bridgeless PFC circuit for power factor correction is shown in FIG. 1. As compared with the conventional PFC circuit including a bridge, shown in FIG. 2, the bridgeless PFC circuit reduces the number of semiconductor devices in the conduction path. In the conventional PFC circuit, there are three semiconductors in the current conduction path. As for the bridgeless PFC circuit, only two semiconductor devices are in any given conduction path. Both circuits work as a boost DC/DC converter, and the switching loss is substantially the same. However, the bridgeless PFC can reduce the circuit conduction loss and improve the circuit efficiency. Further, since the bridgeless circuit only uses two MOSFETs and two diodes, as compared with the conventional PFC, which has one MOSFET and five diodes, the circuit is much simplified.
Although the bridgeless PFC circuit provides simplified circuit structure and improved efficiency, the circuit has severe EMI noise.
In the conventional PFC circuit of FIG. 2, at each half line cycle, the output ground is connected with the input line through a rectifier diode, as shown in FIGS. 3a and 3b, which respectively show the positive and negative half line cycles. Therefore, the only capacitance that contributes to common mode EMI noise is the parasitic capacitor Cd between the MOSFET drain and the earth ground, as shown in FIG. 4.
Additional improvements to bridgeless boost power factor correction circuits are described in Ser. No. 10/953,344, filed Sep. 29, 2004 (IR-2593), incorporated by reference. See also Ser. No. 60/666,950, filed Mar. 31, 2005 (IR-2965 PROV), incorporated by reference.
However, in the bridgeless circuit of FIG. 1, the boost inductor is split and located on the AC side of the circuit. Therefore, the output is not directly connected with the input line. The equivalent circuits of the bridgeless PFC circuit during the positive and negative half line cycle are shown in FIG. 5a and FIG. 5b. The circuit output therefore is floating comparing with the input line and earth ground. Simulation results show that the voltages on the MOSFET drains to the earth ground and thus parasitic capacitors Cd1 and Cd2 (FIG. 6) fluctuate with reverse polarity. And the voltages on the parasitic capacitors Cp and Cn (see FIG. 6) between the output terminal and the earth ground fluctuate with the same polarity. Thus all the parasitic capacitors contribute to common mode EMI noise, as shown in FIG. 7.
Although the common mode noise caused by the parasitic capacitances Cd1 and Cd2 between the MOSFET drains and the earth ground can be reduced or even cancelled by careful design of the parasitic capacitances Cd1 and Cd2 (i.e. by making them symmetrical), the common mode noise caused by the parasitic capacitances Cp and Cn between the output terminals and the earth ground cannot be cancelled. Considering that parasitic capacitance exists not only in the PFC circuit itself, but also in the load of the PFC circuit, common mode noise detriments both the PFC circuit itself and the load of the PFC circuit.
Thus, although the bridgeless PFC circuit was invented around 20 years ago, it hasn't yet been accepted by the industry. Not only does the circuit suffer from severe EMI noise problems, it also has issues of voltage sensing and current sensing. Therefore, for the past 20 years, most of the work has been related to improving the control of the circuit, for example to improve the current sensing and voltage sensing in the bridgeless PFC circuit. One previous circuit has been developed that has improved the EMI performance of the bridgeless PFC circuit. The circuit schematic is shown in FIG. 10. A pair of diodes D3 and D4 are added to the conventional circuit of FIG. 1.
The equivalent circuits of the circuit operating at the positive and negative half line cycles are shown in FIGS. 11a and 11b, respectively. In each half line cycle, the bridgeless PFC circuit works as a boost DC/DC converter. And the overall circuit is equivalent two boost circuits added together. The output voltage ground is tied to the input line through the two diodes D3 and D4. Therefore, this circuit improves the circuit EMI performance and has EMI performance similar to that of the conventional PFC circuit.